EXTERNAL can't refer to NEURON internals.

[1uc]

The way NMODL generates code, we're unable to find a way to have EXTERNAL gbl_foo refer to global variables in other mod files and global variables inside nrn. The issue is that gbl in the mechanism foo isn't implemented as a global double. Instead they're all collected in an struct containing all the global variables (ease of GPU programming).

What makes this difficult is that we can't know if EXTERNAL gbl_foo should be split as {var_name}_{mech_name}. Therefore, we can't deduce which (if any) mechanism gbl belongs. With more help from the DSL we could implement it, e.g.:

EXTERNAL var_name FROM mech_name | "nrn"

The handful of cases I could find on ModelDB never used EXTERNAL to refer to global variables inside nrn, only global variables in other MOD files (under their control).

Therefore, I'd like to propose to remove this part of EXTERNAL. The feature would likely continue to work for NOCMODL, but wouldn't work for NMODL. The tests that check that it's possible to gain access to internal variables would be removed. A similar change was made in the past. When a global variable became a thread-variable, this test was changed to reflect the fact that thread variables can't be the target of an EXTERNAL. This particular example feels like, if it were essential to know the clamp_resist value, it could be handled via the mechanism used for other "NEURON globals" like celsius.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[azure-pipelines]

✔️ 938c3f2 -> Azure artifacts URL

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.10%. Comparing base (ebe3d3b) to head (938c3f2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3165      +/-   ##
==========================================
- Coverage   67.11%   67.10%   -0.01%     
==========================================
  Files         571      571              
  Lines      111318   111318              
==========================================
- Hits        74714    74705       -9     
- Misses      36604    36613       +9     

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[bbpbuildbot]

Logfiles from GitLab pipeline #237474 (:no_entry:) have been uploaded here!

Status and direct links:

• ✅ mac_m1_cmake_build: [cmake, ON, OFF, OFF, address]
• ✅ spack_setup
• ✅ build:nmodl
• ✅ build:neuron:nmodl:intel:legacy
• ✅ build:neuron:nmodl:intel:shared
• ✅ build:neuron:nmodl:nvhpc:acc:legacy
• ✅ build:neuron:nmodl:nvhpc:acc:shared
• ✅ build:neuron:nmodl:nvhpc:omp:legacy
• ✅ build:neuron:nmodl:nvhpc:omp
• ✅ test:neuron:nmodl:intel:legacy
• ✅ test:neuron:nmodl:intel:shared
• ✅ test:neuron:nmodl:nvhpc:acc:legacy
• ⛔ test:neuron:nmodl:nvhpc:acc:shared
• ✅ test:neuron:nmodl:nvhpc:omp:legacy
• ✅ test:neuron:nmodl:nvhpc:omp